1. Field of the Invention
This invention applies to assembling parts in the manufacture of flat tension mask color cathode ray tubes. More specifically, the invention provides means for mapping the path of and positioning an attachment device for affixing a thin tensed foil shadow mask to the mask receiving surface of a mask support structure.
In particular the invention relates to a portion of the process steps employed in the manufacture of the front assembly of a flat tension mask color cathode ray tube. A front assembly includes a flat glass panel, ground flat on one side defined as the inner surface of the panel, a support structure affixed to the inner surface of the panel and a tensed foil shadow mask permanently attached to the support structure with proper registration relative to the color emitting phosphors, applied to the inner surface within the boundaries imposed by the support structure perimeter. Such a front assembly is described in U.S. Pat. No. 4,686,416.
2. Definitions
Certain definitions of terms and phrases used in this disclosure add to the clarity of the description of the invention.
Tube: The term "tube" in this disclosure means a flat tension mask cathode ray tube such as that used as a color television receiver screen or as a color computer monitor screen.
Panel: The front portion of the tube which functions as the viewing screen is a relatively thick flat glass plate.
Mask: An essential part of a tube is a thin metal shadow mask placed close to and behind the inner surface of the panel. In the preferred embodiment of this disclosure, the shadow mask is described as being made of steel and as being permanently affixed to a support structure by means of laser welding.
Support Structure and Rail: For proper functioning of a cathode ray tube, the mask is permanently installed, with proper registration, in a plane fixed at a specified distance from the inner surface of the panel. This specified distance is commonly called the "Q" spacing of the tube and is related to electrical and mechanical geometry of the tube. For the purpose of illustration, the Q spacing of a typical 14" diagonal screen flat tension mask cathode ray tube is approximately 0.290". This dimension is used as the specified distance in the description of the preferred embodiment. The support structure may be part of the panel or made of separate elements attached to the panel. In this document, the metal rectangular structure is called the "support structure". Any one side of the support structure is called a "rail".
Land and Mask Receiving Surface: In preparation for welding, the top surface of the support structure is ground flat. The ground surface of the support structure to which the mask is welded is called the "land" or the "mask receiving surface".
Mapping: The total process of detecting the position of the edges of the mask receiving surface on the support structure, recording its coordinates and computing the exact path to be followed by the welding head is called "mapping".
3. Reference to Prior Art
There is no known prior art related to mapping the coordinates of a path to be followed by an attachment device for the purpose of affixing a tensed foil shadow mask, called a mask, to the support structure of a flat tension mask cathode ray tube.
4. Problems in the Manufacture of Fixed Mask Tubes
A reason for rigid and permanent attachment of the mask to the support structure of a tube is to maintain tension on the mask so it retains its shape and exact registration with the color emitting phosphors deposited on the inner surface of the panel during normal operation of the tube.
What follows is a list of problems addressed and solved by the invention:
A. Unpredictability of Support Structure Position
The support structure is made of a 28% chromium-iron alloy commonly known as "Carpenter Glass Sealing 27". Each of four rails is longitudinally formed into a "V" shape from sheet material approximately 0.024" thick. The four rails are spot welded at the corners of a rectangle. Manufacturing variances contribute to deviations from an ideal rectangle. The rectangular frame with V-shaped cross section is frit sealed to the panel. Experience has shown that it is difficult to bond the support structure to the panel within close tolerances.
B. Variable Width and Height of the Mask Receiving Surface
The available and variable width of the mask receiving surface places constraints on the attachment process. The mask must be precisely installed relative to the inner surface of the panel. The plane of the mask must be parallel to the plane of the inner surface, and it must be a precise distance from the inner surface. Also, it must be registered with respect to the color emitting phosphor dots on the panel in exactly the same way it was registered while the phosphors were being applied.
Variability in the height of the unfinished support structure is caused by the manufacturing processes used to form the metal parts and by the small variances in the fritting process. Height of the support structure is controlled by grinding the narrow edge of the rails to produce a mask receiving surface plane approximately 0.290" (plus or minus 0.002") from the inner surface of the panel.
The grinding process produces a land on the support structure typically 0.06" in width which, in practice, can vary from 0.03" to 0.10". In the process of affixing the mask to the mask receiving surface, a large number of welds, each approximately 0.01" diameter, are placed at intervals of approximately 0.02". For proper attachment of the mask to the support structure, all welds must be placed between the edges of the mask receiving surface.
C. Accessibility of the Path
Fundamental to the art of manufacturing flat tension mask tubes is the requirement that the mask be permanently attached to the panel in exactly the same relative position maintained during all other processes used to make the tube. Further, the requirements of the finished tube demand that the mask be very thin (0.001" thick, for example) and that it be perfectly flat. To assure flatness during assembly and during its normal operation where it encounters significant continuous temperature changes, the mask is retained under tension in two directions throughout the entire tube manufacturing process. This is achieved by clamping the mask in a sturdy but movable frame. During all processes, registration of the mask relative to the panel is maintained by means of ball and groove mechanical indexing devices that repeatedly reproduce proper registration as the separate parts are handled.
This same rigid frame and locating means are used to position the mask during the welding process. When in position for welding, the mask and the frame cause the welding path to be hidden from view and inaccessible to any type of follower system or visually controlled positioning process.
By necessity, the welding path must be defined on the basis of measurements made before the mask is placed in its final position.